[heathkit] need a cheap hi-Z microphone

[G3OOU@aol.com]

Hi John

 

I use a Shure 444 in the high impedance position with my Heath SB101/401 which works fine. I have not measured the audio level that is required by each transmitter but the microphone gives out about 800mV pk-pk on a loud whistle and about 100mV pk-pk on average speech. Most "low" impedance microphones, say 600ohms, give about 10mV pk-pk on average speech levels.

 

A high impedance mic is therefore required to provide sufficient level into the Tx which has an input impedance of 1Meg and a single stage of amplification internally before the balanced modulator.

 

I believe that the 444 has a low impedance mic with a step-up transformer and high-low impedance selector switch in the base. You could try a standard mobile mic with a 10:1 step-up transformer.

 

73

Bob
G3OOU
G-QRP 6907
http://members.aol.com/rfcburns

 

In a message dated 07/12/2007 14:52:07 GMT Standard Time, jjjurek22@att.net writes:

Hi Randy,

 

You wrote:  "Can you let me know the impedance you are looking for so that I am not miss leading you here."

I wish you hadn't brought this up; i'm starting to feel ignorant.

 

I consulted the SB-401 manual, and there is no information in the specs on actual microphone impedance.  All I found was the following:

"It is recommended that a high impedance microphone that is equipped with a push-to-talk switch be used with this transmitter, so advantage may be taken of both the PTT and VOX operation."

 

Looking at the circuit and trying to remember my knowledge of vacuum tube amplifiers, it still does not seem obvious to me why a High impedance is required or recommended.  Yes, the input of the speech amplifier is high impedance to audio frequencies (around 1 megohm) and impedance matching issues come to mind, but this is not a max power transfer situation.  I would think we just want the highest signal voltage possible supplied to the grid, regardless of current, which I presume is very low.  Wouldn't you think it would call for a low microphone impedance for that?

 

Also, I don't see how the impedance of the microphone would affect any dc bias-ing because it is capacitively decoupled.  The vox circuitry is down-stream from the microphone,  and has no interest in microphone impedance, I would think.

 

A few weeks ago I went to a local amateur radio supply store, and the guy said he didn't have a single microphone in the house he could sell me, as they were all low impedance, and recommended I look online for a used Shure 444 or Astatic something.  I suspect that if you used your D-104 on an SB-103 which I presume is tube, then it will work for me on the SB-401.

 

Yes I have paypal.  Give me the postage costs etc. and I'll drop the dough.  I presume this is 35.00 US $?

 

Regards,

John  KC9MJO

 

P.S.

Can anyone out there technically nail down this high-impedance mic issue, just to satisfy my need to know?

 

 

 

----- Original Message -----

From: Randy Elliott

To: heathkit List Member

Sent: Thursday, December 06, 2007 8:10 PM

Subject: [heathkit] need a cheap hi-Z microphone

Hi Again John

Can you let me know the impedance you are looking for so that I am not miss leading you here. I am assuming that your Heath would use the same mic impedance as my SB-102. Also will need your address to figure the shipping rate. I have a Paypal account if that helps.

Randy

On 6-Dec-07, at 8:46 PM, JJJUREK wrote:

Randy,

That sounds good to me.  How do I make payment?

John

----- Original Message -----

From: Randy Elliott

To: heathkit List Member

Sent: Thursday, December 06, 2007 3:02 PM

Subject: [heathkit] need a cheap hi-Z microphone

Hi John

I have an Astatic D-104 that I used on my SB-102, you can that one for 35.00 plus shipping.

Randy Elliott VE3JPU

234 Rands Rd

Ajax, Ontario, Canada

L1S 3Y5

905-427-6853

On 6-Dec-07, at 3:55 PM, Shai Drori wrote:

Any Radio Shack will do then

JJJUREK wrote:

 

I have an SB-401 xmtr for which a need a microphone.  I don't require anything "vintage" quality, just something that works for a reasonable price, like maybe a Shure 444.  Don't care about looks or collectability.  I don't want to pay more than $40 plus shipping.

 

John

KC9MJO